Sphinx is a free open-source SQL full-text search engine.
Generally, it’s a standalone search engine, meant to provide fast, size-efficient and relevant fulltext search functions to other applications. Sphinx was specially designed to integrate well with SQL databases and scripting languages. Currently built-in data sources support fetching data either via direct connection to MySQL or PostgreSQL, or using XML pipe mechanism (a pipe to indexer in special XML-based format which Sphinx recognizes).
Currently, their only indexing the plugin’s title and description and other details from the plugins readme.txt, so this encompasses a good portion in your search for a plugin. I’m pretty sure the search results have improved, because earlier a search for my plugin Contextual Related Posts threw up some random results. Now, there are five results with some relevance.
You still can’t search for plugins based on their author information or tags, those this is soon to be added. However, I’d like to select these in a separate dropdown as we have it in WP-Admin.
If you’re still not happy with the results, you can jump back to the Google Search.
Now, they just need to implement this for the other sections of WordPress.org, especially the forum.